铁磁形状记忆合金Mn2NiGa中应力诱发马氏体相的结构和磁性

采用金刚石对顶砧高压装置(DAC)和同步辐射X射线光源法,对Heusler类型的磁性形状记忆合金Mn2NiGa的结构进行了原位高压X射线衍射测量,并对卸载后的受压样品进行了磁测量.实验观察到材料在室温下分别在0.77 GPa和20 GPa压力下发生了两次不可逆结构相变:马氏体相变和两种不同马氏体间的等结构相变.同时加压使马氏体结构中产生了大量的缺陷,造成了严重的晶格畸变,致使马氏体结构的矫顽力提高了近10倍,达到204 kA/m.结果发现,加压处理造成样品马氏体相饱和磁化强度的大幅度降低,显示出明显的缺陷效应.     

低温时效处理对铁磁形状记忆合金Mn2NiGa的结构、相变和磁性能的影响

对在较低温度范围的时效处理铁磁形状记忆合金Mn₂NiGa的结构、相变和磁性进行了研究.研究发现,母相基体析出了细小的析出相,引起了晶格扭曲和畸变,导致了系统内产生了很大的内应力.在其浓度超过晶格的容忍度之后,提升了体系的马氏体相变温度,使母相在时效温度下转变成马氏体相,并在其中测量到高达900 Oe的矫顽力.由于这种马氏体相的逆相变温度大幅提高,外推获得其居里温度在530 K附近.细小析出相的粗化使内应力消失,样品又回到母相状态.观察到细小析出相粗化的两个阈值温度,分别为423 K和 关键词： 铁磁形状记忆合金 2NiGa')" href="#">Mn₂NiGa 时效处理 内应力     

巨磁阻材料Sr2CrWO6高压下的结构稳定性和电学特性

 采用金刚石压砧高压装置，研究了双钙钛矿结构化合物Sr₂CrWO₆在室温下、34.5 GPa压力内的同步辐射X射线衍射谱，发现在9.6 GPa的压力点样品的结构有所变化。结合室温下20 GPa内电阻和电容随压力的变化，证明样品在9 GPa附近发生了晶体结构相变，而在2~5 GPa的压力范围内样品发生了电子结构相变。     

CaF2纳米晶粒结构相变行为的尺寸依赖性（英文）

利用原位高压同步辐射X射线衍射方法,对尺寸为11 nm的CaF₂纳米晶粒进行高压结构相变和压缩特性研究。当压力为12 GPa时,观察到由萤石结构向α-PbCl₂结构转变的一次相变,该相变压力点远高于体材料,但略低于粒径更小的CaF₂纳米晶体。相比体材料,纳米尺寸的CaF₂样品的体弹模量更大,说明其更难被压缩。当压力释放至常压时,11 nm的CaF₂纳米晶粒的α-PbCl₂型亚稳相结构被保留下来,相变不可逆。分析了影响11 nm CaF₂纳米晶粒独特高压行为的原因,判定尺寸效应为主要因素,该尺寸下较高的表面能导致结构稳定性增强和体积模量增加。     

高压对钴酸锂的晶体结构和离子导电率的影响

采用金刚石对顶砧(DAC)高压产生装置,结合同步辐射X射线衍射(XRD),对钴酸锂(LiCoO2)粉末样品进行室温下原位高压X射线衍射实验,最高压强达到20.3GPa。研究结果表明:在20.3GPa下,LiCoO2的晶体结构非常稳定,并没有发生结构相变;在20.3GPa范围内测量了晶体沿不同晶轴a、c方向的压缩比,发现LiCoO2沿c轴方向的压缩率是沿a轴方向的4.5倍;使用二阶Birch-Murnagha方程拟合出钴酸锂样品的等温状态方程。另外还采用高压原位交流阻抗谱技术(EIS),测量了不同压强下钴酸锂中锂离子导电率,最高压强达到16.8GPa时,发现在实验的压强范围内,随着压强的增加,离子导电率减小。最后将高压下锂离子的电导率与钴酸锂的晶体结构进行联系,进一步阐述了高压下钴酸锂的微观结构和电学性能之间的关系。     

磁性形状记忆合金Mn2NiGa band Jahn-Teller效应的第一性原理研究

运用第一性原理的方法,研究了磁性形状记忆合金Mn₂NiGa在马氏体相变中晶格结构、磁结构、Mn原子d电子结构的变化.研究表明,伴随Mn₂NiGa马氏体相变的发生,形成了一个由两根长键及四根短键组成的拉长八面体结构,即产生了沿z轴拉长的Jahn-Teller畸变;在相变时,位于八面体中心的Mn原子的磁矩发生显著的变化,而作为配体的Ni、Ga原子的磁矩变化很微小;Jahn-Teller畸变的发生,是由于晶体的畸变使配住场产生变化,导致Mn原子d电子态密度重新分布,从而使e_g和t_2g能级分裂所致.     

用SAXS和XRD方法研究TiO2纳米颗粒微观结构

用溶胶-凝胶方法制备了TiO₂纳米样品,并对该样品在300℃到800℃温度区域进行了退火处理.应用同步辐射X射线粉末衍射(XRD)方法研究了经不同热处理温度的TiO₂纳米颗粒的结构相变.应用同步辐射小角X射线散射(SAXS)方法研究了TiO₂纳米颗粒的表面分形与界面特性.得到纳米颗粒粒度与退火温度的变化规律,讨论了表面界面特征与相变的关系. 关键词： X射线小角散射 X射线衍射 2纳米颗粒')" href="#">TiO₂纳米颗粒     

纳米硫化锌球壳的高压相变研究

 采用同步辐射能量色散X射线衍射（EDEX）技术和金刚石对顶砧高压装置,对纳米硫化锌球壳进行了原位高压X射线衍射实验。最高压力达33.3 GPa。常压下纳米硫化锌球壳为纤锌矿结构和闪锌矿结构共存的混相结构。压力达到11.2 GPa时,纳米硫化锌空心球中的纤锌矿结构全部转变为闪锌矿结构。压力达到16.0 GPa时,发生了由闪锌矿结构向岩盐矿结构的相变,在17.5 GPa和21.0 GPa时分别出现未知峰,33.3 GPa时基本完全转变为岩盐矿结构。两个相变均为可逆相变。     

Fe和Co元素在铁磁性形状记忆合金Mn50Ni25－xFe(Co)xGa25中的作用

为了进一步改善材料的性能和探索新的材料，将Mn₂NiGa合金中的Ni元素分别用Fe和Co替代，制备了Mn₅₀Ni_25-xFe(Co)_xGa₂₅系列合金. 研究了Fe和Co元素对Mn₂NiGa合金的结构、马氏体相变行为、磁性和机械性能等方面的影响. 关键词： 铁磁形状记忆合金 Heusler合金 50Ni_25-xFe(Co)_xGa₂₅')" href="#">Mn₅₀Ni_25-xFe(Co)_xGa₂₅     

内应力对Mn2NiGa铁磁形状记忆合金的结构、相变和磁性能的影响

通过施加压应力的方法,在铁磁形状记忆合金Mn₂NiGa中引入残留内应力,研究了内应力对 Mn₂NiGa材料的结构、相变和磁性能的影响.研究发现,加压过程使材料发生了塑性形变,在材料内部引入了大量的位错缺陷.卸载后保留的位错缺陷在材料中造成了残留的内应力,导致了马氏体相变温度大幅度提高, 使原本室温下的母相转变成了马氏体相.测量到导致样品转变成马氏体的阈值压应力为1.0 GPa.加压形成的马氏体中的残留内应力将矫顽力从低于50 Oe提高到350 Oe.残留内应力在730 K的热处理中由于位错缺陷的消失而得以消除,样品实现了马氏体逆相变.如此高的逆相变温度使得 Mn₂NiGa马氏体的居里温度测量成为可能,获得了530K的数值.     

The influence of pressure on spontaneous magnetization of Fe60Mn20B20 amorphous alloy at temperature range 77.4–300 K

Abstract

Amorphous, ferromagnetic, invar like, Fe₆₀ Mn₂₀ B₂₀ alloy has been investigated. Two kinds of experiments were carried out for this alloy. The first, using high pressure technique, revealed the influence of pressure on B(H) dependencies within the wide range of temperature under pressure of 0.5 GPa. From the magnetization curves obtained during these experiments the decrease of spontaneous magnetization caused by applied pressure 0.5 GPa at temperature -180°C has been calculated at the rate about 7 10^?11T/Pa.

In the second kind of experiments the measurements of volume magnetostriction up to 720 kA/m magnetic field intensity have been done. Volume magnetostriction coefficient at temperature 77.4 K has been determined to be about 2 10^?11 [A/m]^?1.     

ELECTRICAL PROPERTIES, EQUATIONS OF STATE AND PHASE TRANSITION IN SOLID C60 AT HIGH PRESSURE

Electrical properties and equations of state in solid C₆₀ at room temperature and high pressure have been studied in a diamond anvil cell using capacitance and resistance measurements and a piston-cylinder type device using P-V measurements, respectively. Experimental results by capacitance and P-V measurements indicate that solid C₆₀ also undergoes a phase transition at room temperature and a lower pressure, about 2GPa. The phase transition may be from a face-centered-cubic to a simple-cubic structure.     

The formation and stability of quasicrystal in Al80Mn14Si6 under high static pressure

Abstract

The method of quenching in fusing state under high static pressure (MQFSHP) was applied for the first time to prepare the quasicrystal icosahedral phase of Al₈₀Mn₁₄Si₆ alloy. The pressure was from 2.8GPa to 3.1GPa and the cooling rate during quenching was of about 100°C/s. Some sharp electron diffraction spots showing an arrangement with a five-fold symmetry axis and noncrystalline ring have been observed in electron diffraction experiment.

The crystallization temperature of I phase obtained from high pressure(HP) is close to that of rapid cooling ribbon, but the cooling rate of the sample obtained is lower than that of rapid cooling ribbon.     

High pressure studies on α-cristobalite form of Al0.5Ga0.5PO4: hydrostatic versus non-hydrostatic conditions

High pressure angle-dispersive X-ray diffraction investigations have been carried out on α-cristobalite form of Al_0.5Ga_0.5PO₄. Our investigations show that the structural stability of this phase under high pressure depends on the nature of pressure conditions in the diamond anvil cell. Under hydrostatic pressure conditions using neon as a pressure transmitting medium, ambient orthorhombic C222₁ phase transforms to orthorhombic Cmcm phase at 4.9?GPa. The high pressure Cmcm phase remains stable up to the highest pressure in the experiment, i.e. 19?GPa. The values of bulk modulus for C222₁ and Cmcm phases are 19(2) and 126(4)?GPa, respectively. In contrast to this, under non-hydrostatic pressure conditions, transformation of ambient C222₁ phase to Cmcm phase has not observed up to 17.4?GPa. Instead, a new monoclinic phase P2₁ is observed which contains layers of six coordinated Al/Ga ions separated by less dense five coordinated Al/Ga ions.     

Pressure-induced change of the interlayer exchange interaction from ferromagnetic to antiferromagnetic in CS2CuF4 observed by neutron scattering experiments up to 2 GPa

Abstract

Neutron scattering experiments on the two-dimensional Heisenberg ferromagnets Cs₂ CuF₄ and K₂CuF₄ have been performed around 2 ～ 3 GPa over 1·4–15 K. At ambient pressure both the intralayer and the interlayer exchange interactions in these two compounds are ferromagnetic. At about 2 GPa, the interlayer exchange coupling in Cs₂ CuF₄ is found to change from ferromagnetic to antiferromagnetic, while the ferromagnetic intralayer exchange interaction is maintained. Contrary to Cs₂CuF₄, the ferromagnetism in K₂Cuf₄ is not destroyed by pressure up to 9 GPa, that was confirmed in the early study of the magnetic susceptibility measurements.     

In situ complex impedance spectroscopy of mantle minerals measured at 20 GPa and 1400°C

Electrical conductivities of mantle silicate minerals (Mg_0·9Fe_0·1)₂SiO₄ olivine, wadsleyite and ringwoodite were determined at pressures up to 20 GPa and temperatures up to 1400°C using complex impedance spectroscopy in a high pressure multianvil apparatus. All samples were polycrystalline, synthesized in separate high pressure experiments prior to the electrical measurements. Olivine conductivities up to 10 GPa are very close to values determined at ambient pressure under controlled oxygen fugacities in previous studies indicating a very small pressure dependence. The conductivities of wadsleyite at 15 GPa and ringwoodite at 20 GPa are similar, and both about 100 times greater than for olivine. When compared to conductivity models of Earth's mantle, these results suggest that the steep increase in conductivity near the transition zone is mainly due to the olivine to wadsleyite phase transformation at 410 km depth, with only minor changes in conductivity occurring over the wadsleyite to ringwoodite transformation near 520 km depth.     

Photoemission study of the (1 0 0) surface of Ni2MnGa and Mn2NiGa ferromagnetic shape memory alloys

The (1 0 0) surface of Ni₂MnGa and Mn₂NiGa ferromagnetic shape memory alloys have been studied by photoelectron spectroscopy and low energy electron diffraction (LEED). It is shown that by sputtering and annealing, it is possible to obtain a clean, ordered and stoichiometric surface that shows a four-fold 1 × 1 LEED pattern at room temperature. For both Ni₂MnGa and Mn₂NiGa, the surface becomes Ni-rich and Mn deficient after sputtering. However, as the annealing temperature is increased Mn segregates to the surface and at sufficiently high annealing temperature the Mn deficiency caused by sputtering is compensated. The (1 0 0) surface of Ni₂MnGa is found to have Mn-Ga termination. The valence band spectra of both Ni₂MnGa and Mn₂NiGa exhibits modifications with surface composition. For the stoichiometric surface, the origin of the spectral shape of the valence band is explained by calculations based on first principles density functional theory.     

Pressure effect of magnetic and electronic properties of Mn2PtGa Heusler alloy

First-principles calculations are performed to investigate pressure effects on structure, magnetism, martensitic phase transition and Curie temperatures of Mn₂PtGa Heusler alloy in framework of the density functional theory. It is shown that Mn₂PtGa prefer to crystallize in the inverse Heusler type structure. Besides, we predict an extraordinary occurrence of pressure induced metallic ferrimagnetism to half-metallic ferromagnetism transition in cubic phase of Mn₂PtGa alloy under hydrostatic pressure up to 43 GPa and the half-metallic ferromagnetism is found to be robust even the lattice further compression to 90 GPa. However, with the pressure up to 100 GPa, the spin-down gap starts to close and the half metallicity begin to disappear, while with the pressure increasing from 100 GPa to 300 GPa, the alloy returns to metallic characteristic. In addition, the energy difference between the austenitic and martensitic phases is found to increase with increasing pressure followed by a decrease when pressure reaches to 43 GPa, which implies a variation trend of martensitic phase transition temperature. Furthermore, Curie temperatures in both austenitic and martensitic phases are estimated under pressure by using the standard mean-field approximation which agrees well with the theoretical results in literature. The robustness of the half metallicity, magnetic transition and the high Curie temperature under pressure make Mn₂PtGa alloy a promising candidate for applications in spintronic devices.